On account of their good mechanical strength, heat resistance, insulating properties, and solvent resistance, polyimides are in general use as an electronic material, such as protective material, insulating material, and color filter, for liquid crystal display units and semiconductors. They are also expected to find new uses as the material of optical waveguide for optical communications and as the material of substrates for mobile phones.
The recent remarkable development in these fields has come to require materials with more sophisticated properties than before. In other words, the polyimide used in these fields needs not only good heat resistance and solvent resistance but also many other properties, such as transparency, for individual applications.
The conventional polyimide in general use is a total aromatic polyimide which is obtained by polycondensation reaction between an aromatic tetracarboxylic dianhydride and an aromatic diamine. Unfortunately, because of its dark amber color, the total aromatic polyimide poses problems in application areas where high transparency is necessary. In practice, the total aromatic polyimide is insoluble in organic solvent, which makes it necessary to form its film from polyamic acid as its precursor by dehydrocyclization with heating.
One way of achieving good transparency is by the polycondensation reaction between an alicyclic tetracarboxylic dianhydride and an aromatic diamine, which yields a polyimide precursor, and the ensuing imidization of the precursor. This process is known to give a highly transparent polyimide with comparatively less discoloration (See Patent Documents 1 and 2).
Unfortunately, the polyamic acid and polyimide formed from an unsubstituted alicyclic tetracarboxylic dianhydride are hardly soluble in ordinary organic solvents and only soluble in high-boiling polar solvents. This necessitates heating at high temperatures for solvent removal in the film-forming process. Heating adversely affects any other organic materials constituting the organic EL element.
There has recently been reported a research on making the gas barrier film for the organic electroluminescence (EL) element from a polyimide polymerized from 1,2,3,4-cyclopentanetetracarboxylic acid-1:2,3:4-dianhydride (CPDA for short hereinafter) (See Patent Document 3).
Unfortunately, this polyimide still has room for improvement in heat resistance because of its low degree of polymerization and is not necessarily satisfactory in solubility in organic solvents.